The present invention is a method and apparatus for printing a document over a network. More specifically, the present invention provides a method and apparatus wherein a host computer generates a raster image from a series of page description language instructions representative of the document, and the raster image is transferred over a network to one or more printers where it is printed.
In the prior art, a document is printed over a network by preparing a series of page description language (PDL) instructions at a host computer and transferring those instructions to a printer over a network. The printer includes a raster image processor (RIP) that processes the series of PDL instructions into a raster image that is stored in a frame buffer, and the printer's print engine prints the raster image from the frame buffer.
Processing the series of PDL instructions into a raster image is known in the art as RIPing, which is a computation intensive and memory intensive process that requires a significant amount of time relative to the amount of time it takes for the print engine to print the image. Therefore, prior art printers that process PDL instructions into raster images generally spend significant amounts of time RIPing the PDL image. During much of this time, the print engine must remain idle waiting for the raster image.
This problem is addressed in U.S. Pat. No. 5,113,494 to Menendez et al., which discloses a high speed raster image processor that RIPs PDL instructions fast enough to minimize the idle time of a laser printer's print engine. The RIP disclosed by Menendez et al. resides in a common printer node with the print engine, and specifies a dedicated connection between the RIP and the print engine. This approach dedicates a significant amount of hardware to perform the RIP function, and increases the cost of the printer.
Another prior approach to this problem is to provide the host computer with a RIP, which is typically implemented by software on the host computer. While this approach does not generally decrease the time required to RIP a document, multiple hosts can RIP documents in parallel, with each host submitting a raster image of a document to a printer's print engine when RIPing is complete. The drawback to this approach, however, is that a tremendous amount of data must be sent over the network. An 81/2.times.11 inch page of color text represented by a sequence of PDL instructions and printed at a resolution of 300 dots per inch (dpi) will generally not require more than twenty kilobytes of data to be transferred over a network. However, a raster image of this same page of color text will require about four megabytes of data to be transferred over a network. In addition, for a printer having a large form factor, such as a 36 inch wide color printer, the amount of data to be transferred makes this approach prohibitive. A 300 DPI color image at a size of 54.times.54 inches requires about 125 megabytes of data to be transferred over the network. Since ripless printers do not contain data storage resources of this magnitude, the raster image must be retransmitted over the network for each printed copy.
On a typical Ethernet network adhering to the IEEE 802.3 specification, raster data cannot be transferred to printer fast enough to feed a moderately fast print engine. In a laser printer, the printer engine must pause between pages to wait for the raster image to be received. In an ink jet printer, the printer may have to pause while printing a page to wait for additional raster data. These pauses may result in a banding effect because the pause may cause the ink deposited during the preceding pass of the print head to dry before the next pass can occur, while most passes will occur continuously and the ink will not dry. When the ink is not dry, the ink deposited between successive passes will blend together and minimize the banding associated with successive passes of the print head, while banding will be more pronounced if the ink has dried before the next pass of the print head occurs.
Another problem associated with RIPing the series of PDL instructions at the host computer is that host computer's RIP will generally lack information about the media and inks of the printer that will eventually print the document. When the RIP is integrated in the printer, the RIP is generally provided with this information.